Where did the saying "You're not just whistling Dixie" come from?

"Dixie", also known as "Dixie's Land", "I Wish I Was in Dixie", is a popular song in the Southern United States. It is one of the most distinctively Southern musical products of the 19th century and probably the best-known song to have come out of blackface minstrelsy. It was not a folk song at its creation, but it has since entered the American folk vernacular. The song likely cemented the word "Dixie" in the American vocabulary as a nickname for the Southern United States.

The song added a new term to the American lexicon: "Whistling 'Dixie'" is a slang expression meaning "engaging in unrealistically rosy fantasizing." For example, "Don't just sit there whistling 'Dixie'!" is a reprimand against inaction, and "You ain't just whistling 'Dixie'!" indicates that the addressee is serious about the matter at hand.